Unity errors when lighting MWI

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I call a subscriber and leave a message. According to what I see Unity can immediately tell when a VM lands in a mailbox. Via status monitor, I can see Unity almost instantaneously attempts to dial the MWI for the ext. For some reason Unity fails to light the lamp. The mwi process normally takes about a second. When watching status monitor I see this taking closer to 30 seconds, at which time Unity writes a failure event to the application log then reattempts the process. Eventually Unity succeeds after several attempts, sometimes it doesn't succeed at all and the lamp is never lit. Any ideas what might cause this?

I have seen this with installing new phones. The problem seems to occur when installing a new phone. The DN receives voice-mail before the phone has registered. I just delete and recreate the voice-mail box since it is using Exchange for the message store.